1. Field of the Invention
The present invention relates to an active vibratory noise control apparatus for supplying an audio signal output from an audio unit and a canceling signal which serves to cancel vibratory noise in a passenger compartment of a vehicle to a common speaker unit, which converts the supplied signals into a reproduced sound.
2. Description of the Related Art
Heretofore, there has been proposed an active vibratory noise control apparatus for supplying an audio signal output from an audio unit and a canceling signal which serves to cancel vibratory noise in the passenger compartment of a vehicle to a common speaker unit, which converts the supplied signals into a reproduced sound (for example, see Japanese laid-open patent publication No. 6-130971).
As shown in FIG. 9 of the accompanying drawings, one example of such an active vibratory noise control apparatus has an active vibratory noise control unit 30-3, an audio unit 70, and a speaker unit 41.
The audio unit 70 has a sound source device 49 and an adder circuit 51. One audio sound source is selected from the sound source device 49, and an audio signal output from the selected audio sound source is supplied to the adder circuit 51.
The speaker unit 41 has an amplifier 42 and a speaker 43 disposed in the passenger compartment. A signal output from the adder circuit 51 is amplified by the amplifier 42, which supplies an output signal to the speaker 43 to convert the signal into a reproduced sound.
The cancellation of vibratory noise produced by an engine as a vibratory noise source, e.g., vibratory noise produced in the passenger compartment of a vehicle by the rotation of a 4-cycle 4-cylinder engine, will be described by way of example below. The 4-cycle 4-cylinder engine produces vibrations due to torque variations thereof upon gas combustion each time the engine output shaft makes one-half of a revolution, causing vibratory noise in the passenger compartment of the vehicle. The 4-cycle 4-cylinder engine produces a lot of vibratory noise that is referred to as a rotational secondary component having a frequency which is twice the rotational speed of the engine output shaft.
In the active vibratory noise control unit 30-3, a basic signal generating circuit 2 generates a basic signal which is a digital signal having a frequency selected from the frequencies of vibratory noise generated by a vibratory noise source, and an adaptive filter 4 generates a canceling signal which serves to cancel vibratory noise in the passenger compartment based on the basic signal. A reference signal generating circuit 5-1 corrects the basic signal from the basic signal generating circuit 2 based on corrective data depending on signal transfer characteristics to generate a reference signal. A microphone 27 disposed in the passenger compartment detects an error signal based on the vibratory noise in the passenger compartment. Based on the reference signal from the reference signal generating circuit 5-1 and the error signal, an LMS algorithm processing circuit 6 calculates filter coefficients of the adaptive filter 4 so as to minimize the error signal, and successively updates the filter coefficients of the adaptive filter 4 for the adaptive filter 4 to generate a canceling signal to minimize the error signal.
The basic signal generating circuit 2, the adaptive filter 4, the reference signal generating circuit 5-1, and the LMS algorithm processing circuit 6 are implemented by a microcomputer 20-3.
The canceling signal generated by the adaptive filter 4 is added to the audio signal output from the sound source device 49 by the adder circuit 51, which outputs a sum signal to drive the speaker unit 41. Therefore, the speaker unit 41 for generating a reproduced sound based on the audio signal output from the audio unit 70 doubles as a speaker unit for generating a canceling sound based on the canceling signal output from the active vibratory noise control unit 30-3.
The signal transfer characteristics referred to above range from the adaptive filter 4 to the LMS algorithm processing circuit 6. The active vibratory noise control unit 30-3 corrects the basic signal using the corrective data based on the signal transfer characteristics, and generates the canceling signal matching the signal transfer characteristics from the adaptive filter 4.
For measuring actual signal transfer characteristics of the active vibratory noise control apparatus, as indicated by the broken lines in FIG. 9, a signal transfer characteristics measuring circuit 100 comprising a Fourier transform device is connected between the output terminal of the adaptive filter 4 and the error signal input terminal of the LMS algorithm processing circuit 6. The signal transfer characteristics measuring circuit 100 measures signal transfer characteristics between the output terminal of the adaptive filter 4 and the error signal input terminal of the LMS algorithm processing circuit 6 across the passenger compartment.
Therefore, the measured signal transfer characteristics include signal transfer characteristics due to a D/A converter 21, a low-pass filter 22, the adder circuit 51 and an amplifier 42 which are connected from the output terminal of the adaptive filter 4 to the speaker 43, and an amplifier 23, a bandpass filter 24, and an A/D converter 25 which are connected from the microphone 27 to the LMS algorithm processing circuit 6.
Audio devices for use on vehicles include audio devices having an ordinary configuration (also referred to as audio devices of standard specifications) where an audio unit is mounted in the instrument panel of a vehicle and a speaker unit comprises an amplifier and speakers that are located in predetermined positions in the passenger compartment of the vehicle, and audio devices based on premium specifications (also referred to as audio devices of premium specifications) where an audio unit is mounted in the instrument panel of a vehicle and a speaker unit comprises an equalizer, an amplifier, and speakers that are located in predetermined positions in the passenger compartment of the vehicle for producing high-fidelity, high-power playback audio signals. Audio devices of different configurations are incorporated in different vehicles depending on different vehicle types and classes.
As a result, signal transfer characteristics in vehicles which incorporate different audio devices are different as indicated by the broken- and solid-line curves as shown in FIGS. 10A and 10B of the accompanying drawings, based on the audio devices. FIG. 10A shows gain characteristics in the signal transfer characteristics, and FIG. 10B show phase characteristics in the signal transfer characteristics. The broken-line curves represent the characteristics of an audio device of standard specifications, and the solid-line curves represent the characteristics of an audio device of premium specifications.
Therefore, it is necessary to provide a plurality of active vibratory noise control units, each sharing the speaker unit with the audio device in use for matching signal transfer characteristics, depending on the speaker units of audio devices that are available for use.
As a consequence, there need to be an increased number of combinations of audio devices for use on vehicles and active vibratory noise control units, posing a problem in that active vibratory noise control units of wrong types may possibly be installed in combination with audio devices on vehicles.